Power distribution units have long been utilized to supply power to electronic equipment. A conventional power-distribution unit (PDU) is an assembly of multiple electrical “outlets” (also called “receptacles” or “outputs”) that receive electrical power from a source and distribute the electrical power via the outlets to one or more separate electronic equipment units having respective power cords plugged into respective outlets of the PDU. In some applications, a PDU receives power from two different power inputs, commonly referred to as “dual feed” or “dual input” PDUs. Such dual inputs can provide additional power supply capability to a PDU, and/or may provide redundant sources of power for equipment that receives power from PDU outlets. PDUs can be used in any of various applications and settings such as, for example, in or on electronic equipment racks (such as RETMA racks) to provide power to network devices (e.g., servers, routers, gateways, network switches), among other applications. One or more PDUs located in a cabinet may for convenience be referred to as a Cabinet Power Distribution Unit (CDU).
Power distributed to small businesses or residential customers is commonly “single phase” or “dual phase” power. In a single phase system, a single alternating current with a sinusoidal voltage is distributed through a two-line connection. In a split phase system, two alternating voltages are distributed through at least three lines: one neutral line and one other line for each of the two phases. The two voltage waveforms voltages are separated in time by a “phase difference” of 180 degrees—that is, the sinusoidal form of the voltage on one line leads or lags the sinusoidal form of the voltage on the other line by the amount of the phase differential. The effective voltage between the first phase line and the second phase line is therefore significantly greater than the effective voltage between each of the phase lines and the neutral line. As a result, a three-line, split-phase system may provide, for example, 120 volts in a phase-to-neutral line circuit and 240 volts in a phase-to-phase line circuit.
In larger commercial and industrial applications, three phase systems may be employed. In three phase systems, each voltage cycle on each phase line is 120 degrees, or ⅓ of a period, out of phase with the voltage cycle on each of the other two phase lines. Three phase systems are used in large commercial and industrial applications because three-phase equipment is smaller in size, weighs less, and is more efficient than single or dual phase equipment. Although three phase circuits are somewhat more complex than single or dual phase circuits, they weigh less than single phase circuitry for the same loads supported by the circuitry. Three phase circuits also can provide a wide range of voltages and can be used for single or dual phase loads.
Three phase power is generated by circuits in either of two configurations: (i) a “delta”; or (ii) a “wye” configuration. If one end of each of the legs of a three-phase circuit is centrally connected at a common point and the other ends are connected to three phase lines (one line for each phase), the configuration is called a wye or “Y” connection. If the legs of the three phase circuits are connected instead in series to form a closed loop, with one phase line connected to each junction of two adjacent legs, the configuration is called a delta or “Δ” connection.
One reason that three phase circuits are more complex than typical single phase circuits is the need to maintain at least somewhat balanced loads among each of the three phases. One indicator of imbalance is the level of current flowing through each phase line. If the level of current flowing through a phase line is significantly different than that flowing through a different phase line, the load is considered to be unbalanced. In a wye connected system, imbalance can also be indicated by current flowing through the neutral line. Imbalance between the loads can result in damage to the three phase system, can cause excessive wear of components in the system such as the three-phase generator, can result in increased power usage, and can be difficult and costly to correct.
For example, high capacity data centers used in computer and communications network applications commonly utilize three-phase power for provide operating power to equipment located in hundreds or thousands of equipment racks within the data center. Commonly, three-phase power is supplied to the equipment racks via a four or five line input, providing a line for each voltage phase, an earth ground, and a neutral line for three-phase wye connections. A vertically, or horizontally, oriented power distribution unit connects to the input and distributes power of differing phases to a plurality of outputs for the phase. A three-phase PDU typically provides three or more branches of outputs, one branch for each phase of power provided by the three-phase plug strip. The PDU can be mountable on or adjacent to a given equipment rack in order to supply three or more branches single phase power (with each such branch derived from the three-phase power input) to the rack or other equipment in the vicinity.